Space-like proton form factors have been measured
with very high precision by many experiments [1]}, [2]}.
In the time-like region, there have been a few measurements
of \(G_{eff}\) , by DM2 [3]}, [4]}, E760 [5]},
PS170 [6]}, FENICE [7]}, E835 [8]}, [9]},
BaBar [10]}, [11]} and CMD-3 [12]}, [13]},
but these have relatively poor precision and mutual agreement.
For \(|G_E/G_M|\)  ratio, the measurements were rare and
there is a long-time tension between PS170 and BaBar.
The BESII experiment also measured the proton effective form factor,
but with poor statistical precision [14]}.
BESIII continued this effort using the 2012 and 2015 scan data,
and produced the most accurate \(|G_E/G_M|\)  ratio measurements
at 16 c.m. energies between 2.0 and 3.08 GeV [15]}, [16]}
that favor BaBar over PS170 and helped clarifying the puzzle.
BESIII also performed the measurements using the ISR
technique [17]}, [18]},
with results that are consistent with BaBar's.
The BESIII measurements are shown in Fig. REF 
(a) for \(p\bar{p}\)  production cross section in \(2.0 - 3.08\)  GeV,
(b) the effective proton time-like form factor,
(c) the form factor ratio \(R=|G_E/G_M|\) , and
(d) the effective form factor residual,
together with results from other experiments.
The best precision in the time-like region was reported by BESIII,
and the electric form factor was extracted for the first time.
The unprecedented 3.5% uncertainty that was achieved at 2.125 GeV
by BESIII is close to that of the best measurements in the space-like
region, which have been at per cent level for a long time.
The CMD-3 experiment measured the production cross section of proton pair
and observed an abrupt rise at the nucleon-antinucleon threshold [13]},
as expected for point-like charged particles according to Eqn. REF .
BESIII did not extend down to the threshold energy, but the results
around 2 GeV agree with CMD-3.
This information improves our understanding of the proton inner structure
from a different dimension and helps to test theoretical models
that depend on non-perturbative QCD, e.g. charge distribution
within the proton can be deduced [20]}, [21]}.
The near threshold behavior of the electromagnetic form factor
of a hadron is mostly determined by the interaction of the
hadron-antihadron in the final state, and therefore the measurements
of the form factor properties can also serve as a fruitful source of
information about hadron-antihadron interaction [22]}.
